My friend came to visit me and during the flight, his transponder, a Garmin 45R, was not working.* On the ground, it is showing "Transponder Fault '' on G3X right above the squawk*code.* We could change squawk code and he said his ADSB was working fine as he could see traffic.* Power cycling*did not do anything and we checked the wiring at the transponder and all looked to be OK.